Among currently employed processes for synthesizing acetic acid, one of the most useful commercially is the catalyzed carbonylation of methanol with carbon monoxide as taught in U.S. Pat. No. 3,769,329, herein incorporated by reference. The carbonylation catalyst contains a metal catalyst, such as rhodium, which is either dissolved or otherwise dispersed in a liquid reaction medium or supported on an inert solid, along with a halogen-containing catalyst promoter as exemplified by methyl iodide. The rhodium can be introduced into the reaction system in many forms. Likewise, because the nature of the halide promoter is not generally critical, a large number of suitable promoters, most of which are organic iodides, may be used. Most typically and usefully, the reaction is conducted by continuously bubbling carbon monoxide gas through a liquid reaction medium in which the catalyst is dissolved.
During the carbonylation of methanol, by-products are formed. These by-products may include alkanes and alkane-like materials. U.S. Pat. No. 5,371,286 teaches a method to improve the quality of certain residues by modifying the separation of alkanes and alkane-like materials and carbonyl-containing impurities from the recycle during the manufacture of acetic acid by the carbonylation of methanol. The improvement in U.S. Pat. No. 5,371,286 comprises operating a prior art stripper column in a reflux manner and partitioning the residue therefrom by the addition of water. The method in U.S. Pat. No. 5,371,286 also reduces the volume of disposal of potentially valuable recyclable reactants. Several steps are claimed in U.S. Pat. No. 5,371,286, including separating a volatile component by causing the volatile component to reflux in a reflux column, removing the volatile portion for further processing, separating a residue into two layers by the addition of water, and recycling an aqueous layer to the reactor. Such a process is referred to herein as an alkane removal system.
An additional by-product formed during the carbonylation of methanol is acetaldehyde. Reduction of acetaldehyde has been described in the art. For example, U.S. Pat. No. 5,756,836 teaches a process for producing a highly purified acetic acid characterized in that the process comprises the step of continuously reacting methanol and/or an aqueous solution of methyl acetate with carbon monoxide in a reactor. A treatment is conducted to limit the concentration of unsaturated compounds in crude acetic acid obtained in the process to 5 wppm or lower, and the resultant crude acetic acid is ozonized. The '836 patent also teaches a process for producing a highly purified acetic acid, characterized by the step of continuously reacting methanol and/or an aqueous solution of methyl acetate with carbon monoxide in a reactor while maintaining the concentration of acetaldehyde in a reaction fluid in the reactor at 1500 wppm or lower. The acetaldehyde concentration is controlled by conducting said reaction at a water content not greater than 10 wt. % and an acetaldehyde concentration of not greater than 1500 wppm to produce a crude acetic acid product mixture; sending the crude acetic acid product mixture to a distillation column to produce a high-boiling point fraction and a low-boiling point fraction; treating the low-boiling point fraction to reduce the content of acetaldehyde therein; and returning the treated low-boiling point fraction to the reaction system.
U.S. Pat. No. 5,625,095 also suggests that acetaldehyde concentration should be reduced. The '095 patent discloses a high purity acetic acid prepared by reacting methanol with carbon monoxide in the presence of a rhodium catalyst, iodide salts, and methyl iodide, wherein an acetaldehyde concentration in the reaction liquid is maintained at 400 wppm or lower. This may be attained by contacting the liquid containing carbonyl impurities with water to separate and remove the carbonyl impurities. After that, the liquid can be returned to the reactor.
U.S. Pat. No. 6,573,403 teaches a process for producing acetic acid which comprises charging reactants methanol, dimethyl ether, methyl acetate or any mixture thereof into a reactor containing: (1) a rhodium carbonylation catalyst, (2) an alkyl iodide or alkyl bromide, and (3) a hydrogenation catalyst, and contacting the reactants with carbon monoxide and hydrogen to produce acetic acid. The '403 patent further teaches that the addition of ruthenium compounds to the carbonylation reaction solution conditions effectively reduces the formation of undesired carbonyl impurities whilst increasing the formation of ethanol, ethyl acetate and ethyl iodide being precursors for the formation of valuable propanoic acid.
Additional methods for removing permanganate reducing compounds (PRC's), including acetaldehyde, are disclosed in U.S. Pat. Nos. 7,855,306 and 7,683,212. The '306 patent teaches a process for reducing and/or removing permanganate reducing compounds or their precursors from intermediate streams during the formation of acetic acid by said carbonylation processes. In particular, a process in which a low boiling overhead vapor stream from a light ends column is subjected to a single distillation to obtain an overhead that is subjected to an extraction to selectively remove and/or reduce PRC's from the process is disclosed. Although alkane removal systems as described herein were able to remove alkanes, they were no longer necessary with the introduction of a permanganate removal system as described in U.S. Pat. No. 7,855,306 since the removal of acetaldehyde reduced the formation of the alkanes. The '212 patent teaches a method to produce acetic acid by continuously reacting methanol with carbon monoxide in the presence of a rhodium catalyst, an iodide salt, methyl iodide, methyl acetate, and water; and thereby producing acetic acid at a production rate of 11 mol/L·hr or more while keeping the acetaldehyde content of a reaction mixture to 500 wppm or less, in which the reaction is carried out at a carbon monoxide partial pressure in a gaseous phase of a reactor of 1.05 MPa or more and/or at a methyl acetate content of the reaction mixture of 2 percent by weight or more to thereby keep the production rate of acetaldehyde to 1/1500 or less that of acetic acid. The '212 patent teaches that this method can reduce production of by-products without reducing the reaction rate of acetic acid even at a low water content and a low hydrogen partial pressure in a reaction system.
Although the above-described publications focus on suppressing or removing carbonyl impurities such as alkanes, alkane-like materials, and acetaldehyde from carbonylation reaction systems, little art exists concerning the removal or control of the formation of ethyl iodide, which can be formed from these impurities, especially in the absence of an alkane removal process. The need therefore exists for improved processes for producing a reaction medium comprising low amounts of ethyl iodide.